Linear induction motor

ABSTRACT

A linear induction motor has propulsive force produced in the electrically conductive reaction member forming part or all of its secondary member by flux which passes in the primarysecondary magnetic structure in both transversely orientated and longitudinally orientated magnetic laminations.

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LINEAR INDUCTION MOTOR Inventors: John Frederick Eastham, Long Ditton;Hugh Robert Bolton, London, both of England Assignee: Tracked HovercraftLimited, London, England Filed: Apr. 4, 1972 Appl. No.: 241,069

Foreign Application Priority Data Sept. 18. I969 Great Britain..46074/69 Related US. Application Data Continuation of Ser. No. 73,107,Sept. 17, 1970, abandoned.

US. Cl. 310/13, 104/148 LM Int. Cl. I-I02k 91/09 Field of Search310/12-14; 318/135;

orientated magnetic laminations.

[ Nov. 6, 1973 [5 6] References Cited UNITED STATES PATENTS 3,135,8796/1964 Baumann 310/13 3,370,191 2/1968 Koch 310/13 X 2,993,130 7/1961Laithwaite.. 310/13 3,356,041 12/1967 Bliss 104/148 Primary Examiner-D.F. Duggan Attorney-Cameron, Kerkam & Sutton [57] ABSTRACT A linearinduction motor has propulsive force produced in the electricallyconductive reaction member forming part or all of its secondary memberby flux which passes in the primary-secondary magnetic structure in bothtransversely orientated and longitudinally 15 Claims, 11 Drawing FiguresPATENTED Nov 6 I975 SHEET t CF 4 LINEAR INDUCTION MOTOR This is aContinuation, of application Ser. No. 73,l07, filed Sept. 17, 1970, nowabandoned.

The transverse flux paths are preferably of a substantially lower a.c.reluctance than the longitudinal paths, so that the majority of thepropulsive force is produced by the transversely directed flux. Theamount of magnetic material needed for the primary and, if appropriate,secondary members is then substantially independent of the pole pitch ofthe energising winding so that long pole pitches can be used.

The invention has particular application to high speed transportationsystems such as systems in which air cushion vehicles operate at highspeeds along prepared tracks.

This invention relates to linear induction motors, that is to say, toelectric induction motors having a primary and secondary member arrangedtransversely of one another for relative longitudinal movement. Theprimary member comprises magnetic material formed with an energisingwinding, and the secondary member comprises an electrically conductivereaction member which may or may not be backed by a magnetic backingmaterial according to whether the motor is of the single-sided kind orof the double-sided kind.

According to the present invention a linear induction motor comprises aprimary member and a secondary member arranged transversely of oneanother for relative longitudinal movement therebetween, said primarymember comprising magnetic material, and winding means formed on saidmagnetic material and arranged, when energised from an alternatingcurrent supply, for creating a field of magnetomotive force whichtravels longitudinally of the motor, said secondary member comprisingelectrically conductive material, said primary and secondary membersbeing arranged and disposed transversely of the motor to provide incombination a first plurality of flux paths which are orientatedtransversely of the motor and which pass through said electricallyconductive material, and a second plurality of flux paths which areoriented longitudinally of the motor and which also pass through saidelectrically conductive material, in operation flux driven around theflux paths of both pluralities by the winding means inducing in theelectrically conductive material currents which react with that flux toproduce a longitudinally directed force between the primary andsecondary members.

Preferably the reluctances of the transversely orientated paths and ofthe longitudinally orientated paths are such that the longitudinallyorientated paths provide only a small proportion (e.g., of the order ofpercent) of the propulsive force. The motor then relies to a greatextent on transversely directed flux for the production of propulsiveforce. This in turn means that the amount of magnetic material whichmust be provided for the primary and, if appropriate, secondary membersis'substantially independent of the pole pitch ofthe travelling field ofmagnetomotive force set up by the energising winding. The motor is thusparticularly suited to applications in which large pole pitches arerequired to give high synchronous speeds at low (e.g., 50 Hz)frequencies.

The longitudinal oreintated laminations and hence the longitudinal fluxpaths) may be provided for various reasons which differ according to thekind of linear induction motor involved. The energising winding may beformed on longitudinally spaced stacks of transverse laminations, inwhich case the longitudinal laminations encourage the secondary membercurrents to pass transversely of the motor in the positions at whichthey are effective to produce propulsive force. Alternatively thelongitudinal laminations may themselves carry the energising winding, inwhich case they allow semiclosed slots for the winding to be readilyprovided, en abling conventional winding techniques to be used, andallow the motor width and height to be reduced.

In the above paragraph as in the remainder of specification and claimsthe word reluctance" is used in its complex, i.e., A.C. sense. In thecontext of the present invention it is used in relation to flux pathswhich are provided by the primary-secondary magnetic structure, and willbe understood to be dependent upon the frequency of the flux reversalsin the flux path to which it refers because of the eddy currentsassociated with the flux path.

In order that the invention may be more fully understood, a number ofembodiments of the invention will now be described by way of example,with reference to the accompanying diagrammatic drawings in which:

FIG. 1 is a perspective view showing a typical part of the firstembodiment,

FIG. 2 is a central vertical section along a typical part of the motorof FIG. 1,

FIG. 3 is a perspective view showing a typical part of the secondembodiment,

FIG. 4 is a perspective view showing a typical part of the thirdembodiment,

FIG. 5 is a central vertical section along a typical part of the motorof FIG. 4,

FIGS. 6, 7, 8, 9 and 10 are perspective views showing a typical part of,respectively, the fourth, fifth, sixth, seventh and eighth embodiments,and

FIG. 11 shows transverse section through an air cushion vehiclepropelled along a track by a linear induction motor according to theinvention.

The linear induction motor shown in FIGS. 1 and 2 has a primary member 2and secondary member 4 arranged transversely of one another for relativelongitudinal movement in the direction of the arrow P. For conveniencethe primary member is described and shown as being vertically above thesecondary member, but it will be appreciated that in this, as in theother embodiments of the invention, any orientation may be used.

The primary member 2 has a number of discrete stacks 6 of mutuallyinsulated laminations 8 of magnetic material each orientatedtransversely of the motor. The lamination stacks 6 are twelve in number(only three being shown for clarity) and are spaced regularly apartlongitudinally of the motor.

The laminations 8 are E-shaped so that each of the stacks has threeparallel limbs of equal length. The outer ones of these limbs areindicated by the reference numeral 9 and the centre ones by thereference numeral 7. The centre limbs are approximately twice as wide asthe outer limbs. The stacks 6 are each disposed so that the limibs 7, 9extend downwardly to their end faces 3.

Extending between each pair of successive stacks 6 at the root of thecentre limbs 7 are stacks 12 of longitudinally orientated and mutuallyinsulated laminations 13 of magnetic material. Although the longitudinallaminations 13 are shown in FIGS. l and 2 to be horizontal, they couldbe vertical or inclined if desired.

The secondary member 4 is an elongate member of uniform cross sectionwhich comprises a plate-like reaction member Zil, of aluminium or otherelectrically conductive and preferably non-magnetic material, backed bymagnetic backing material 22 formed of transversely orientated andmutually insulated magnetic laminations 23.

The reaction member 20 is a shallow inverted channel having its thin,web portion 24 opposite the centre limbs 7 of the stacks 6. From theportion 24 the reaction member extends trasversely of the motor to stopjust short of the outer limbs 9 of the stacks at its thicker sideportions 26.

The laminations 23 extend beneath the reaction member and project beyondit at either side as to oppose the end faces 3 of the outer limbs 9 ofthe stacks 6 at horizontal top faces ill. The faces H and the topsurface of the reaction member 20 form a plane upper surface it) for thesecondary member 4.

Encircling the centre limbs 7 of the stacks 6 in a common horizontalplane are simple multiturn coils l4, one to each centre limb. Thesetwelve coils 14 are connectable by winding connections (not shown) tothe three phases of a three-phase a.c. supply. Denoting the three phasesby the colours Red (R), yellow (Y) and Blue (B), successive coils Malong the primary member are connected to the phases in the order R, V,B, B, Y, B, R, B, It, 1, & B where the bar over a letter denotes areversed connection.

With the coils energised in this manner the threephase winding whichthey form generates, in known manner, a field of magnetomotive forcewhich travels longitudinally of the motor at a speed determined by thespacing of the stacks 6 and the frequency of the a.c. supply. The twelvecoils M are such as to provide four magnetic poles of this travellingfield, but the number of stacks 6 and hence coils M can, of course, bevaried to reduce or increase the number of magnetic poles.

The laminations 8 of the stacks 6 together with the underlying secondarymember laminations 23 provide low reluctance paths which are orientatedtransversely of the motor. Two such paths are provided side by sideacross the width of the motor and, as is indicated in FIG. 1 by thebroken lines 116, each coil 14 drives flux in opposite senses around thepairs of side-by-side low reluctance paths associated with itsrespective stack.

Flux passing in each low reluctance path in this way crosses the air gapbetween the primary and secondary members twice, once beneath the centrelimb 7 and once beneath the respective outer limb 9.

In crossing the air gap beneath the centre limbs 7 the flux created bythe coils M passes through the web portion 241 of the reaction memberand in so doing induces in the reaction member currents which flow inthe plane of the reaction member in generally rectangular pathscorresponding to the magnetic poles set up by the primary member. Inknown manner these currents react with the flux crossing the air gapbeneath the centre limbs 7 to produce a longitudinally directed forcebetween the primary and secondary members, as indicated in FIG. 1 by thearrow P. The thickened, side portions 26 of the reaction member providelow resistance paths for the longitudinally directed parts of thecurrent paths, and by so doing ensure that beneath the cenre limbs 7 thecurrents are directed substantially transversely of the motor and so areeffective to produce propulsive force. An additional reason for theprovision of the side portions 26 is that they enable the longitudinallydirected parts of the current paths to be substantially free of the fluxcrossing the air gap beneath the centre limbs 7, so that little or noforce tending to move the primary and secondary members laterally of oneanother is created.

In addition to the transversely orientated low reluctance pathsdescribed above the primary-secondary magnetic circuit also providesflux paths which are directed longitudinally of the motor. Referring inparticular to FIG. 2 it will be seen that the stacks 112 oflongitudinally orientated laminations provide part of longitudinal fluxpaths which are further provided by the laminations 8 of the two stacks6 between which the stack 12 extends, and the laminations 23 of thesecondary member. Two such longitudinal flux paths are indicated in FIG.2 by the broken lines 1%.

By providing the stacks l2 and hence the associated longitudinal fluxpaths, transverse currents flowing in the reaction member between thecentre limbs 7 in the manner indicated in FIG. 2 by the referencenumeral 28 are caused to see a high inductive impedance. The transverseparts of the secondary member current paths are therefore encouraged topass in the reaction member beneath the centre limbs 7 where they areeffective to produce propulsive force. In this way the propulsive forceproduced by the motor is to some extent increased.

It will be appreciated that the longitudinally directed flux pathsprovided as described above will have a substantially higher reluctancethan the transversely orientated paths previously described. This isbecause they are transverse to the laminations 23 of the secondarymember and also to the laminations 8 of the primary member. It isenvisaged that the combined reluctance of the longitudinally orientatedpaths provided in part by a stack 12 will be of the order of ten timesthe reluctance of each of the two side-by-side paths (comprising amultiplicity of paths in parallel) provided in part by each stack 6.

In addition to encouraging the transverse parts of the secondary membercurrent paths to pass beneath the centre limbs 7 as described above, thelongitudinally orientated flux paths will have a further effect inincreasing the propulsive force above that produced by the transverseflux alone. This is by virtue of flux which is driven around thelongitudinal flux paths by the three-phase winding and which willaugment the transverse flux where it crosses the air gap between thecentre limbs 7 and the secondary member, thereby producing additionalpropulsive force. However, because the longitudinal paths have arelatively high reluctance, the contribution of the longitudinal flux tothe total propulsive force will be small, e.g., of the order of 10percent.

The second embodiment of the invention, shown in FIG. 3, is the same asthe first embodiment in many respects and like reference numerals areused to indicate like parts. The second embodiment differs from thefirst embodiment in that the separate stacks 12 of longitudinallaminations 13 are replaced by a single stack 30 of longitudinallyorientated and mutually insulated vertical laminations 31 which extendscontinuously along the primary member at the top of the laminationstacks 6. The width of the lamination stack 3'0 is the same as that ofthe stacks 6. If desired the laminations 31 may be disposed in threediscrete stacks one for each limb 7 and 9 and corresponding laterallythereto.

The secondary member 4 of this second embodiment differs from that ofthe first embodiment in that the reaction member 34, instead of stoppingshort of the outer limbs 9, extends across the magnetic laminations 23of the secondary member to project beyond the side edges of thesecondary member magnetic material 35 at overhanging portions 39.

Beneath the outer limbs 9 the reaction member is denoted by thereference numeral 36 and has the same thickness as the portion 38corresponding to the web portion 24 of the first embodiment. Likewise,the overhanging portions 39 have the same, greater, thickness as theportions 33 corresponding to the side portions 26 of the firstembodiment.

The second embodiment operates in substantially the same manner as thefirst embodiment except that secondary member current paths providingpropulsive force are associated with the outer limbs 9 as well as withthe centre limbs 7. In an analogous manner to the first embodiment, thelaminations 31 serve to provide, in part, longitudinally directed fluxpaths which constrain the transverse parts of the secondary membercurrents to pass beneath the limbs 7 and 9 and which also providepropulsive force; one such flux path between two successive outer limbs9 is indicated in FIG. 3 by the broken line 32. The overhanging portions39 and the portions 33 serve the same function as the side portions 26of the first embodiment.

In a non-illustrative modification of the first embodiment the secondarymember is as shown and described for the second embodiment and thelaminations 13 are of the same width as the stacks 6. Likewise in anonillustrated modification of the second embodiment the secondarymember is as shown and described for the first embodiment and thelaminations 31 extend only across the width of the centre limbs 7.

In further non-illustrated modifications of the first and secondembodiments and the above modifications thereof, each limb 7 and 9carries a coil 14, for each stack 6 the outer coils 14 being energisedfrom the same phase as the centre coil so as to aid the centre coil todrive flux around the side-by-side transversely orientated lowreluctance paths provided.

In a non-illustrated embodiment of the invention the primary membermagnetic material comprises discrete stacks oftransversely orientatedlaminations. The lamination stacks are spaced apart longitudinally ofthe motor, and transversely of the motor are generally U- shaped, havingtwo limbs which extend toward the secondary member. The polyphasewinding means is formed on the limbs of the lamination stacks along atleast one side of the motor.

The primary member magnetic material further includes longitudinallaminations which may be arranged in discrete stacks each of whichextends between the opposed faces of successive ones of the stacks oftransverse laminations (as in FIGS. 1 and 2) or they may be arranged inone or more stacks each of which extends continuously along the primarymember adjacent the parts of the stacks of transverse laminations remotefrom the secondary member (as in FIG. 3).

The secondary member comprises an aluminum reaction member backcd bymagnetic material formed of transversely orientated laminations. Thereaction member may oppose the limbs of each stack of transverselaminations along only one side of the primary member or it may opposeboth limbs of each stack. Preferably the reaction member has thickenedportions (e.g., 26,33,39) along the sides of the or each part thereofwhich is opposed to the limbs of the primary member stacks of transverselaminations.

In the arrangements so far described the polyphase winding is carried bytransversely orientated laminations; in the further embodiments of theinvention still to be described with reference to the drawings, thepolyphase winding is carried by longitudinally orientated laminations.

In the third embodiment of the invention, shown in FIGS. 4 and 5, theprimary member 40 comprises a stack 44 of mutually insulated andlongitudinally orientated vertical magnetic laminations 46. The stack 44extends continuously along the primary member.

Transverse winding slots 50 are regularly formed in the stack 44 at itsunder surface, and in these slots are received the winding conductors ofa two-layer threephase distributed winding of the kind which is wellknown in the linear induction motor art. In FIGS. 4 and 5 the windingconductors of this winding, i.e., the parts of the winding received inthe winding slots 50, are indicated by the reference numeral 51 and thewinding ends by the reference numeral 52.

Transversely orientated laminations 54 each generally in the form of aninverted U back the laminations 46 with their arms 62 extendingdownwardly outside the winding ends 52 to end faces 64. The end faces 64and the end faces 48 of the magnetic teeth 45 between the winding slots50 are generally coplanar.

' The secondary member 42 for co-operation with the primary member 40 isessentially the same as the secondary member 4 of the first embodiment,and like ref erence numerals are used to indicate like-parts. In thisthird embodiment the free top faces 11 of the laminations 23 oppose theend faces 64 of the laminations 54 and the web portion 24 of thereaction member 20 opposes the end faces 48 of the magnetic teeth 45.

For operation the winding is energised from a threephase a.c. supply andin known manner generates a field of magnetomotive force which travelslongitudinally of the motor. The transverse laminations 54 and 23 andthe longitudinal laminations 46 provide in combination low reluctancepaths which are orientated transversely of the motor and of which twoare provided side-by-side across the width of the motor. In the mannerpreviously described the travelling field of magnetomotive force drivesflux around these paths (as is illustrated in FIG. 4 by the broken linesand creates propulsive force by interaction with currents which itinduces in the reaction member 20.

In addition to providing part of the low reluctance paths as describedabove, the laminations 46 in combination with the laminations 23 of thesecondary member provide further paths for magnetic flux which areorientated longitudinally of the motor. Flux is driven around theselongitudinally orientated paths by the three-phase winding asillustrated in FIGS. 4 and 5 by the broken lines 68, and this fluxaugments the transversely directed flux where it crosses the air gapbetween the stack 44 and the secondary member and so increases thepropulsive force above that provided by the transverse flux. However,because the longitudinally directed flux paths are transverse of thelaminations 23 and, in addition, the parts of the laminations 46 abovethe winding slots 50 are of such a depth that they saturate at a lowlevel of flux, the reluctance of the longitudinal paths will besubstantially higher than that of the transverse paths previouslydescribed and the contribution of the longitudinal flux to the totalpropulsive force will be correspondingly small (e.g., of the order of 10percent). The main reasons for the provision of the longitudinallydirected laminations 46 to carry the three-phase winding are to enablethe winding slots 50 readily to be partly closed if desired by suitablywidening the bases of the magnetic teeth 45 in the longitudinaldirection, to allow conventional winding techniques to be used to formthe three-phase winding before the transverse laminations 54 are added,and to allow the width and height of the motor to be reduced because thereduction in the flux required to be carried by the laminations 54enables their width in the transverse direction to be correspondinglyreduced.

FIG. 6 shows the fourth embodiment of the invention to have a primarymember 7i comprising two lamination stacks each of which is identical tothe lamination stack 44 of the third embodiment, both in respect of itsmagnetic structure and also in respect of the threephase winding withwhich it is formed; the same reference numerals as the third embodimentare therefore used for these two stacks and their associated windings.

The two stacks 44 are arranged side-by-side with a longitudinal gap 77between them and with their winding slots 50 aligned. Discrete stacks 78of mutually insulated transverse laminations 79 are disposed on top ofthe stacks 4 and are of a length to correspond laterally to the totalwidth of those stacks. Longitudinally of the motor the stacks 7%correspond to the underlying magnetic teeth 45 of the stacks 44.

The secondary member 31 for co-operation with this primary member is ofuniform cross-section and comprises an aluminium reaction member 82backed by a magnetic backing member 33. The member 83 is formed ofgenerally U-shaped, transversely orientated and mutually insulatedmagnetic laminations 84 and has its upwardly extending armscorresponding laterally to the stacks 44.

The reaction member 82 has a plane upper surface and extends across thewidth of the backing member 83 to project beyond it on either side atoverhanging portions 85. These overhanging portions 85 and the portion86 beneath the slot 77 are of the same equal thickness which is greaterthan the thickness of the portions 87 beneath the stacks 44.

In operation the two three-phase windings are energised to producetravelling fields of magnetomotive force which are in antiphase to oneanother transversely of the motor. The two windings therefore additivelycombine to drive flux around transversely orientated low reluctancepaths which are provided as indicated in FIG. 6 by the broken line 88.It will be seen that each such path comprises laminations from thestacks 73 and 83 and from the stacks 44. It will also be seen that,whereas in the third embodiment (FIGS. 4 and two transversely orientatedlow reluctance paths are provided side-by-side across the width of themotor, in the embodiment of FIG. 6 only one transversely orientated lowreluctance path is provided across the width of the motor.

In the manner previously described, the transversely orientated fluxcreates propulsive force by interaction with current which it induces inthe reaction member 82. It will be appreciated that propulsive forcewill be generated beneath both stacks 44 and that the portions and 86 ofthe reaction member serve the same function as the reaction memberportions 26, 33 and 39 previously mentioned.

Additional propulsive force is also produced by interaction of reactionmember currents with flux which passes in longitduinally orientatedpaths provided by each stack 44 in combination with the underlyingsecondary member laminations 8d. One such flux path is indicated in FIG.6 by the broken line 92. As in the third embodiment the contribution ofthe longitudinal flux to the total propulsive force will be small, e.g.,of the order of 10 percent.

FIG. 7 shows a fifth embodiment of the invention to be a modification ofthe embodiment of FIG. 6 with only one of the stacks 441 of the primarymember 90 slotted and formed with a winding. The other, unslotted, stackis designated by the reference numeral 91 and is formed of rectangular,longitudinally orientated and mutually insulated magnetic lamenations89. The transverse laminations 96 of the primary member are shown toextend continuously along the primary member, but it will be appreciatedthat, as in FIG. 6, discrete lamination stacks disposed above themagnetic teeth 45 can be used if desired.

The reaction member 93 of the secondary member 94 is associated onlywith the wound stack 44, stopping just short of the unwound stack 91 atthe inner one of its thickened portions 95. In addition the backingmember 99, formed of transversely orientated and mutually insulatedmagnetic laminations 75, has its free upper face corresponding laterallyto the lamination stack 91 and coplanar with the upper plane face of thereaction member 93.

In a modification of this fifth embodiment the secondary member 81 ofFIG. 6 is used.

The sixth embodiment of the invention, shown in FIG. 8, is a furthermodification of the embodiment of FIG. 6 with one of the stacks 44 ofthe primary member Mill replaced by transversely orientated and mutuallyinsulated magnetic laminations 98 which, like the stack 91 in FIG. 7,carry no winding. As shown in FIG. 8, the laminations 98 which arealigned with the magnetic teeth 45 of the wound stack 3 1 extend abovethe stack 44 to terminate vertically above its outer face in laminationstacks 97; the laminations 98 aligned with the winding slots 50,however, are rectangular and essentially act as spacers. In amodification the latter laminations are replaced by non-magneticspacers.

The secondary member of this sixth embodiment is identical to thesecondary member 94 of the fifth embodiment and like reference numeralsare therefore used to indicate like parts.

The invention has so far been described in relation to single-sidedlinear induction motors but may also be applied to linaer inductionmotors of the double-sided kind, that is to say, linear induction motorsin which the primary member is in two interconnected parts which inoperation are disposed on either side of the cooperating secondarymember. The secondary member itself is comprised ofa plate-like memberof electrically conductive and preferably non-magnetic material such asaluminium.

Referring now to FIG. 9, the primary member 101 of the seventhembodiment of the invention has two lamination stacks which areidentical to the stacks 44 of the embodiments of FIGS. 4 to 8, and thesame reference numerals are therefore used to indicate like parts.

Each stack 44 is carried with its laminations 45 horizontal by amagnetic yoke member 102 formed of transversely orientated and mutuallyinsulated magnetic laminations 103. In transverse cross-section the yokemember is generally in the form of an inverted U, having two verticaland downwardly depending arms 104 to the inside faces of which thelamination stacks 44 are respectively attached so as to oppose oneanother at their inner faces 118 provided by the magnetic teeth 45.

The opposed faces 118 are separated by a gap through which extends, inspaced relation, the secondary member 106 of the linear induction motor.This secondary member 106 is provided by a vertical platelike member ofaluminium which is mounted at its base at an integral foot portion 107and which extends vertically between and beyond the stacks 44 toterminate at an enlarged portion 108. Between the stacks 44 the member106 is of constant thickness.

For operation, the windings of the two stacks 44 are energised from athree-phase a.c. supply so as to create travelling fields ofmagnetomotive force which are in antiphase transversely of the motor,and these two travelling fields combine additively to drive flux aroundtransversely orientated low reluctance paths which are provided asillustrative in FIG. 9 by the broken line 124. It will be seen thatthese low reluctance paths are provided in part by the laminations 46 ofthe stacks 44 and in part by the laminations 103 of the yoke member 102.

In crossing the gap between the stacks 44 the fiux in these transverselow reluctance paths passes through the member 106 and induces in themember currents which flow in the plane of the member in generallyrectangular paths corresponding to the magnetic poles of the travellingfields of magnetomotive force. The interaction of the flux crossing theair gap and the transverse (i.e., vertical) component of these secondarymember currents produces a desired longitudinal force between theprimary and secondary members.

Those parts ofthe member 106 which project beyond the laminations 46 inthe vertical direction serve to provide the low reluctance pathspreviously mentioned for the longitudinal directed parts of thesecondary member currents. It is for this reason that the enlargedportion 108 is provided.

In addition to producing the transversely directed flux as describedabove, the travelling fields of magnetomotive force also drive fluxaround the primarysecondary magnetic structure in the longitudinal(horizontal) direction, as is illustrated in FIG. 9 by the broken line122.

Flux passing in these longitudinal paths augments the flux in thetransverse paths and so contributes to the propulsive force. Thecontribution will, however, be small (e.g., of the order of 10 percent)because the longitudina] paths are transverse to the laminations 103 andalso because, as previously described in relation to the thirdembodiment, the depth of the laminations 46 where they bridge thewinding slots 50 is such that saturation occurs at a low level of flux.

FIG. 10 shows the eighth embodiment of the invention to be amodification of the embodiment of FIG. 9 with one of the stacks 44omitted and replaced by inward extensions of the laminations 103 at thecorresponding arm 104 of the yoke member 102. These inward extensionsform the yoke with a shoulder 105 which opposes the inner face 118 ofthe stack 44 at its own inner face 120.

The secondary member of this embodiment, which is identical to thesecondary member 106 of the embodiment of FIG. 9 and is therefore giventhe same reference numerals, extends in spaced relation between theopposed faces 118 and 120.

The operation of the embodiment of FIG. 10 is identical to that of theembodiment of FIG. 9 except that only one three-phase winding isprovided for driving flux around the transversely orientated flux paths124 and the longitudinally orientated flux paths 122.

In each of six further, non-illustrated double-sided linear inductionmotors in accordance with the invention the primary member is formed oftwo of the primary members of a respective one of the first sixembodiments aligned face-to-face and interconnected by a non-magneticyoke member. The secondary member is a generally plate-like membersimilar to the member 106 of FIGS. 9 and 10 and which extends in spacedrelation between the two primary member parts. The three-phase windingsof the two primary member parts are energised to produce travellingfields of magnetomotive force which are in anti-phase transversely ofthe motor.

FIG. 11 shows a gas cushion vehicle arranged for operation along aconcrete track 142 of rectangular cross section. The vehicle haslongitudinally spaced and flexibly mounted gas cushion devices 144 forsupporting it above the track by co-operation with the horizontal topsurface 151 of the track. Likewise the vehicle is guided along the trackby flexibly mounted gas cushion devices 146 co-operating with thevertical side surfaces 153 of the track.

The vehicle is propelled along the track by a linear induction motorwhich may be any one of the first six embodiments previously described.The primary member 1148 of the motor is carried by the vehicle forcooperation with the secondary member which is an elongate memberextending along the track and inset centrally into the top track surface151. Servoactuators 152 enable the primary member 148 to be maintainedsubstantially at a pre-determined position relative to the secondarymember 150 in both the vertical and lateral directions, despitesubstantial transverse movements of the vehicle body relative to thetrack.

In a non-illustrated modification of FIG. 11, the vehicle is propelledalong the track by one of the embodiments shown in FIGS. 9 and 10. Theplate-like reaction member 106 is centrally mounted on the top tracksurface 151, so as to form a vertical spine along the track. The vehicleis shaped to accommodate the reaction member and carries the primarymember 101 for cooperation with the reaction member by means ofservoactuators which enable the relative disposition of the primary andsecondary members to be maintained substantially constant.

It will be noted that in all the described embodiments of the inventionthe primary-secondary magnetic structure is such that flux which crossesthe air gap between the primary and secondary members so as to passthrough the electrically conductive reaction member partly flows inpaths which are directed transversely of the motor and partly flows inpaths which are directed longitudinally thereof. In all the embodimentsof the invention the longitudinal paths have a reluctance which,although small in itself, is substantially higher than the lowreluctance of the transverse paths, with the result that thecontribution of the longitudinally directed flux to the propulsive forcewill be small. The motors described with reference to the drawingstherefore rely to a great extent on transversely directed flux for theproduction of propulsive force, and in that respect the depth of themagnetic material required of the primary and secondary members for agiven flux density will be substantially independent of the pole pitchof the polyphase winding formed on the primary member. This is becausethe cross-sectional area of the path through which the transverse fluxpasses in a longitudinally directed plane perpendicular to the generalplane of the air gap between the primary and secondary members, beingthe product of the pole pitch and the core depth, is proportional topole pitch. By core depth is meant the depth of the primary membermagnetic teeth minus the teeth or limbs (as appropriate), or of thesecondary member magnetic material if provided.

The feature that at least a substantial proportion of the propulsiveforce is generated by transversely orientated flux therefore means thatthe motors of the described embodiments are advantageous in applicationswhere long pole pitches are necessary, since a large pole pitch willnot, as would otherwise be the case, necessitate an excessive bulk (andhence cost) of magnetic material for the primary and, where appropriate,secondary members. This feature is particularly advantageous forsingle-sided linear induction motors since the magnetic material for themember which forms part of the track, usually the secondary member,represents a considerable proportion of the cost of the system as awhole.

The longitudinally orientated laminations, and hence the longitudinalflux paths, are provided in the described embodiments for variousdifferent reasons which have already been mentioned. In the first twoembodiments the longitudinally orientated laminations are provided forconstraining the laterally extending parts of the secondary membercurrent paths to pass beneath the primary member limbs where they areeffective to produce propulsive force; in the remaining embodiments ofthe invention two longitudinal directed laminations are provided for,amongst other reasons, enabling the winding slots to be readily madesemi-closed and conventional winding techniques to be used, and forenabling the width of the motor to be reduced.

It is not essential for a motor in accordance with the invention to beso arranged that, as in the described embodiment, a large majority ofthe propulsive force is produced by the flux which passes in thetransversely orientated paths; a motor in accordance with the inventionmay, in fact, be such that the propulsive forces separately produced bythe longitudinal and transverse fluxes are in any finite ratio whichwill, of course, depend on the relative values of the reluctances of thelongitudinal and transverse flux paths. Thus within the scope of theinvention are linear induction motors which are essentially longitudinalflux machines, that is to say, they essentially rely on longitudinalflux for producing propulsive force, but which, fo some reason oranother, ahve flux paths provided for transverse flux.

Although the primary member magnetic material of the describedembodiments is laminated both longitudinally (i.e., to form transverselaminations) and transversely, in some applications of the inventioneither one or both of the parts of the primary member magnetic materialserving to provide at least part of the transverse and longitudinal fluxpaths may be unlaminated.

We claim:

11. A linear induction motor comprising a primary member and a secondarymember spaced apart with a single planar air gap therebetween, saidprimary member comprising magnetic material and winding means formed onsaid magnetic material and arranged, when energised from an alternatingcurrent supply, for creating a field of magnetomotive force whichtravels longitudinally of the motor, said secondary member comprisingelectrically conductive material, said primary and secondary membersbeing arranged and disposed transversely of the motor to provide incombination a first plurality of flux paths which are orientatedtransversely of the motor and a second plurality of flux paths which areorientated longitudinally of the motor, in operation flux driven aroundthe flux paths of both pluralities by the winding means inducing in theelectrically conductive material currents which react with that flux toproduce a longitudinally directed force between the primary andsecondary members.

2. A linear induction motor comprising a primary member and a secondarymember spaced apart with a single planar air gap therebetween, saidprimary member having magnetic material comprising a first part and asecond part and winding means formed on said first part and arranged,when energised from an alternating current supply, for creating a fieldof magnetomotive force which travels longitudinally of the motor, saidsecondary member comprising electrically conductive material, saidprimary and secondary members being arranged and disposed transverselyof the motor to provide in combination a first plurality of flux pathswhich are orientated transversely of the motor, and a second pluralityof flux paths which are orientated longitudinally of the motor, the fluxpaths of one of said plurality being provided in part by the said firstpart of the primary member magnetic material and in part by the saidsecond part thereof, the flux paths of the other said plurality beingprovided in part by only the first part of the primary member magneticmaterial, in operation flux driven around the flux paths of bothpluralities by the winding means inducing in the electrically conductivematerial currents which react with that flux to produce a longitudinallydirected force between the primary and secondary members.

3. A linear induction motor comprising a primary member and a secondarymember spaced apart with a single planar air gap therebetween, saidprimary member having magnetic material comprising a first part formedof at least one stack of magnetic laminations and a second part, andwinding means formed on said first part and arranged, when energisedfrom an alternating current supply, for creating a field ofmagnetomotive force which travels longitudinally of the motor, saidsecondary member comprising electrically conductive material, saidprimary and secondary members being arranged and disposed transverselyof the motor to provide in combination a first plurality of flux pathswhich are orientated transversely of the motor and which pass throughsaid electrically conductive material, and a second plurality of fluxpaths which are orientated longitudinally of the motor and which alsopass through said electrically conductive material, the flux paths ofone said plurality being provided in part by said first part of theprimary member magnetic material and in part by the said second partthereof, the flux paths of the other said plurality being provided inpart by only the said first part of the primary member magneticmaterial, in operation flux driven around the flux paths of bothpluralities by the winding means inducing in the electrically conductivematerial currents which react with that flux to produce a longitudinallydirected force between the primary and secondary members.

4. A linear induction motor comprising a primary member and a secondarymember spaced apart with a single planar air gap therebetween, saidprimary member having magnetic material comprising a first part formedof at least one stack of magnetic laminations each orientatedlongitudinally of the motor and a second part formed of magneticlaminations each orientated transversely of the motor, and winding meansformed on one of said parts and arranged, when energised from analternating current supply, for creating a field of magnetomotive forcewhich travels longitudinally of the motor, said secondary membercomprising electrically conductive material, said primary and sec ondarymembers being arranged and disposed transversely of the motor to providein combination a first plurality of flux paths which are orientatedtransversely of the motor and which pass through said electricallyconductive material, and a second plurality of flux paths which areorientated longitudinally of the motor and which also pass through saidelectrically conductive material, the flux paths of one said pluralitybeing provided in part by the said first part of the primary membermagnetic material and in part by the said second part thereof, the fluxpaths of the other said plurality being provided in part by only thesaid part of the primary member magnetic material which is formed withthe winding means, in operation flux driven around the flux paths ofboth pluralities by the winding means inducing in the electricallyconductive material currents which react with that flux to produce alongitudinally directed force between the primary and secondary members.

5. A linear induction motor according to claim 3, wherein the first partof the primary member magnetic material comprises a plurality oflamination stacks which are spaced apart longitudinally of the motor andwhich are formed with the winding means, each lamination stack beingformed of transversely orientated laminations and having at least twotransversely spaced limbs extending towards the secondary member, thesecond part of the primary member magnetic material being. arrangedmagnetically to bridge the said spaces between the lamination stacks.

6. A linear induction motor according to claim 5, wherein the secondpart of the primary member magnetic material comprises discrete portionseach disposed to extend between the opposed faces of successive ones ofthe lamination stacks.

7. A linear induction motor according to claim 5, wherein the secondpart of the primary member magnetic material extends continuously alongthe primary member adjacent the lamination stacks.

8. A linear induction motor according to claim 5, wherein the secondarymember includes magnetic material formed of transversely orientatedmagnetic laminations arranged to provide further parts of the flux pathsof each said plurality.

9. A linear induction motor according to claim 2, wherein the first partof the primary member magnetic material extends continuously along theprimary member and has its face opposed to the secondary member formedwith longitudinally spaced winding slots in which are received thewinding conductors of the winding means, the first part of the primarymember magnetic material providing part of the flux paths of the saidsecond plurality and the second part of the primary member magneticmaterial extending transversely of the motor from the said first part toprovide, in combination with the said first part, a least part of theflux paths of said first plurality.

10. A linear induction motor according to claim 9, wherein the firstpart of the primary member magnetic material is dimensioned to saturateat a low level of flux where it bridges the winding slots.

11. A linear induction motor according to claim 9, which includes twosaid first parts of the primary member magnetic material spacedtransversely apart by a gap which is magnetically bridged by the saidsecond part of the primary member magnetic material, the winding meansof the two said first parts being energisable to produce travellingfields of magnetomotive force which are substantially in antiphase withone another transversely of the motor.

12. A linear induction motor according to claim 9, wherein the secondpart of the primary member magnetic material extends transversely of themotor from either side of the first part to provide, in combination withthe first part, part of the flux paths of two said first pluralitiesside-by-side across the width of the motor.

13. A linear induction motor according to claim 11, wherein the saidfirst parts of the primary member magnetic material are arrangedside-by-side with their said faces formed with the winding slotsgenerally coplanar and facing in the same direction, and the secondarymember comprises a generally plate-like member of electricallyconductive material generally parallel to the plane of said faces, andfurther magnetic material adjacent the side of the said member remotefrom the primary member for providing part of the flux paths of thefirst and second pluralities.

114. A linear induction motor according to claim 11, wherein the secondpart of the primary member magnetic material is generally U-shaped incross-section having two generally parallel arms, and the two said firstparts of the primary member magnetic material are carried by respectiveones of the arms of the insides thereof so as to oppose one another inspaced relationship at their said faces formed with the winding slots,the secondary member being constituted by said electrically conductivematerial which extends between the opposed faces in spaced relationthereto.

15. A linear induction motor as claimed in claim 1 in combination with avehicle which carries the primary member and a prepared track alongwhich the vehicle is arranged for operation and which carries thesecond: ary member, the primary and secondary members being arranged forco-operation to propel the vehicle along the track.

1. A linear induction motor comprising a primary member and a secondarymember spaced apart with a single planar air gap therebetween, saidprimary member comprising magnetic material and winding means formed onsaid magnetic material and arranged, when energised from an alternatingcurrent supply, for creating a field of magnetomotive force whichtravels longitudinally of the motor, said secondary member comprisingelectrically conductive material, said primary and secondary membersbeing arranged and disposed transversely of the motor to provide incombination a first plurality of flux paths which are orientatedtransversely of the motor and a second plurality of flux paths which areorientated longitudinally of the motor, in operation flux driven aroundthe flux paths of both pluralities by the winding means inducing in theelectrically conductive material currents which react with that flux toproduce a longitudinally directed force between the primary andsecondary members.
 2. A lInear induction motor comprising a primarymember and a secondary member spaced apart with a single planar air gaptherebetween, said primary member having magnetic material comprising afirst part and a second part and winding means formed on said first partand arranged, when energised from an alternating current supply, forcreating a field of magnetomotive force which travels longitudinally ofthe motor, said secondary member comprising electrically conductivematerial, said primary and secondary members being arranged and disposedtransversely of the motor to provide in combination a first plurality offlux paths which are orientated transversely of the motor, and a secondplurality of flux paths which are orientated longitudinally of themotor, the flux paths of one of said plurality being provided in part bythe said first part of the primary member magnetic material and in partby the said second part thereof, the flux paths of the other saidplurality being provided in part by only the first part of the primarymember magnetic material, in operation flux driven around the flux pathsof both pluralities by the winding means inducing in the electricallyconductive material currents which react with that flux to produce alongitudinally directed force between the primary and secondary members.3. A linear induction motor comprising a primary member and a secondarymember spaced apart with a single planar air gap therebetween, saidprimary member having magnetic material comprising a first part formedof at least one stack of magnetic laminations and a second part, andwinding means formed on said first part and arranged, when energisedfrom an alternating current supply, for creating a field ofmagnetomotive force which travels longitudinally of the motor, saidsecondary member comprising electrically conductive material, saidprimary and secondary members being arranged and disposed transverselyof the motor to provide in combination a first plurality of flux pathswhich are orientated transversely of the motor and which pass throughsaid electrically conductive material, and a second plurality of fluxpaths which are orientated longitudinally of the motor and which alsopass through said electrically conductive material, the flux paths ofone said plurality being provided in part by said first part of theprimary member magnetic material and in part by the said second partthereof, the flux paths of the other said plurality being provided inpart by only the said first part of the primary member magneticmaterial, in operation flux driven around the flux paths of bothpluralities by the winding means inducing in the electrically conductivematerial currents which react with that flux to produce a longitudinallydirected force between the primary and secondary members.
 4. A linearinduction motor comprising a primary member and a secondary memberspaced apart with a single planar air gap therebetween, said primarymember having magnetic material comprising a first part formed of atleast one stack of magnetic laminations each orientated longitudinallyof the motor and a second part formed of magnetic laminations eachorientated transversely of the motor, and winding means formed on one ofsaid parts and arranged, when energised from an alternating currentsupply, for creating a field of magnetomotive force which travelslongitudinally of the motor, said secondary member comprisingelectrically conductive material, said primary and secondary membersbeing arranged and disposed transversely of the motor to provide incombination a first plurality of flux paths which are orientatedtransversely of the motor and which pass through said electricallyconductive material, and a second plurality of flux paths which areorientated longitudinally of the motor and which also pass through saidelectrically conductive material, the flux paths of one said pluralitybeing provided in part by the said first part of the primary membermagnetic material and in part by the said seCond part thereof, the fluxpaths of the other said plurality being provided in part by only thesaid part of the primary member magnetic material which is formed withthe winding means, in operation flux driven around the flux paths ofboth pluralities by the winding means inducing in the electricallyconductive material currents which react with that flux to produce alongitudinally directed force between the primary and secondary members.5. A linear induction motor according to claim 3, wherein the first partof the primary member magnetic material comprises a plurality oflamination stacks which are spaced apart longitudinally of the motor andwhich are formed with the winding means, each lamination stack beingformed of transversely orientated laminations and having at least twotransversely spaced limbs extending towards the secondary member, thesecond part of the primary member magnetic material being arrangedmagnetically to bridge the said spaces between the lamination stacks. 6.A linear induction motor according to claim 5, wherein the second partof the primary member magnetic material comprises discrete portions eachdisposed to extend between the opposed faces of successive ones of thelamination stacks.
 7. A linear induction motor according to claim 5,wherein the second part of the primary member magnetic material extendscontinuously along the primary member adjacent the lamination stacks. 8.A linear induction motor according to claim 5, wherein the secondarymember includes magnetic material formed of transversely orientatedmagnetic laminations arranged to provide further parts of the flux pathsof each said plurality.
 9. A linear induction motor according to claim2, wherein the first part of the primary member magnetic materialextends continuously along the primary member and has its face opposedto the secondary member formed with longitudinally spaced winding slotsin which are received the winding conductors of the winding means, thefirst part of the primary member magnetic material providing part of theflux paths of the said second plurality and the second part of theprimary member magnetic material extending transversely of the motorfrom the said first part to provide, in combination with the said firstpart, a least part of the flux paths of said first plurality.
 10. Alinear induction motor according to claim 9, wherein the first part ofthe primary member magnetic material is dimensioned to saturate at a lowlevel of flux where it bridges the winding slots.
 11. A linear inductionmotor according to claim 9, which includes two said first parts of theprimary member magnetic material spaced transversely apart by a gapwhich is magnetically bridged by the said second part of the primarymember magnetic material, the winding means of the two said first partsbeing energisable to produce travelling fields of magnetomotive forcewhich are substantially in antiphase with one another transversely ofthe motor.
 12. A linear induction motor according to claim 9, whereinthe second part of the primary member magnetic material extendstransversely of the motor from either side of the first part to provide,in combination with the first part, part of the flux paths of two saidfirst pluralities side-by-side across the width of the motor.
 13. Alinear induction motor according to claim 11, wherein the said firstparts of the primary member magnetic material are arranged side-by-sidewith their said faces formed with the winding slots generally coplanarand facing in the same direction, and the secondary member comprises agenerally plate-like member of electrically conductive materialgenerally parallel to the plane of said faces, and further magneticmaterial adjacent the side of the said member remote from the primarymember for providing part of the flux paths of the first and secondpluralities.
 14. A linear induction motor according to claim 11, whereinthe second part of the primary member magnetic material is generaLlyU-shaped in cross-section having two generally parallel arms, and thetwo said first parts of the primary member magnetic material are carriedby respective ones of the arms of the insides thereof so as to opposeone another in spaced relationship at their said faces formed with thewinding slots, the secondary member being constituted by saidelectrically conductive material which extends between the opposed facesin spaced relation thereto.
 15. A linear induction motor as claimed inclaim 1 in combination with a vehicle which carries the primary memberand a prepared track along which the vehicle is arranged for operationand which carries the secondary member, the primary and secondarymembers being arranged for co-operation to propel the vehicle along thetrack.